(1) Field of the Invention
This invention relates to a process for imparting a lower reflectance and a higher transmittance to a transparent shaped article.
(2) Description of the Prior Art
Reduction of the reflectance of a transparent shaped article and increase of the transmittance thereof are very important for effective utilization of rays of light and elimination of blurring of images occurring due to reflected images, and many methods have heretofore been proposed for attaining the reduction of reflectance and the increase of transmittance.
The principle of these proposed methods resides in that an optical thin film composed mainly of an inorganic substance having a refractive index different from that of a substrate is formed on the surface of the transparent substrate, to achieve reduction of the reflectance and increase of the transmittance. As means for enhancing this effect, there have been adopted a method in which a plurality of thin films differing in the refractive index are formed on a substrate by multi-coating procedures, a method in which the thicknesses of respective thin films are varied depending upon the wavelengths of corresponding rays of light, and a method in which a so-called optically heterogeneous film is formed on the surface of the transparent substrate, which film has a refractive index continuously varying through the thickness thereof.
For example, in the case of the method where a single anti-reflective thin film is formed on the surface of a substrate, it has been admitted that it is preferable that the anti-reflective thin film to be formed on the surface of the substrate be composed of an inorganic substance having a refractive index as low as possible, such as magnesium fluoride, and the optical thickness of the anti-reflective thin film be adjusted to 1/4 of the wavelength of the objective ray of light.
Substrates to which such anti-reflective thin film can be applied are restricted by the process for forming the anti-reflective thin film. The substate to which such anti-reflective thin film has been most popularly applied is a glass substrate. The technique of coating a thin film of an inorganic substance on the surface of such a glass substrate is difficult to apply to other substrates of different materials or of a large size because many limitations are imposed.
As the above coating technique, there can be mentioned a vacuum evaporation deposition method, a sputtering method for improving the adhesion and an electron beam method. However, it is difficult to apply these coating methods to plastic materials which have recently been popularly used in the field of spectacle lenses and to plastic films and sheets on which anti-reflective thin films can advantageously be formed. Various problems arise when these coating methods are applied especially to plastic materials having a high-hardness coating formed thereon for improving the scratch resistance.
More specifically, plastic materials are ordinarily poor in heat resistance and they cannot resist the above-mentioned coating process, and such troubles as thermal degradation, melting thermal deformation and production of optical strain are often caused. Furthermore, the adhesion is ordinarily poor in plastic materials. These disadvantages are mainly due to the difference of the expansion coefficient between a plastic material and an inorganic substance to be coated thereon. If the adhesion is extremely reduced when the plastic material is exposed to an elevated temperature or a high humidity, cracks and other defects are often formed on the inorganic substance coating layer.
A more serious problem is how to eliminate a phenomenon in which the impact resistance and flexibility of a plastic material are drastically reduced by formation of such an inorganic substance coating layer. Namely, the superiority of plastic materials to glass materials is lost by the presence of such coating, and this is quite a serious problem.